In a papermaking machine, it is now common to incorporate a shoe press. A shoe press comprises a press roll and a shoe opposed to the press role. A wet paper web is typically carried, between felts, through a nip formed between the shoe and the press roll. A shoe press apparatus has a larger nip area than a press comprising a pair of opposed press rolls.
In FIG. 1(a), which is a schematic view of a conventional shoe press apparatus 100a, a relatively long shoe press belt B is used. In the shoe press apparatus 100b, of FIG. 1(b), a relatively short shoe press belt is used. Each of these shoe presses has a press part P, comprising a press roll R and a shoe S. A wet paper web W, disposed between a pair of felts F, passes, along with the belt B, through the press part P as the press roll R rotates in the machine direction, depicted by arrow MD.
FIG. 2 is a schematic view of a conventional shoe press 100c, used in the calender part of a papermaking machine. A calender belt BC and a rough paper web W′ are sandwiched in the press part P between a calender roll R′ and a shoe S as the calender roll R′ rotates.
The belts B and BC are used in different parts of the papermaking machine and their structures are different in detail, in order to achieve the desired effects. However, their basic structure is the same, in that they both comprise a base body for imparting strength to the belt, and a high molecular weight elastic section, which formed on the base body. The high molecular weight elastic section prevents liquid from passing from one surface of the belt to the opposite surface.
A lubricant, which in many cases is oil, is usually supplied to the shoe press apparatus to reduce friction between the shoe and the belt. Reduction of the friction between the shoe and the belt reduces the energy required in order for the press roll to drive the belt.
A problem in the operation of a conventional shoe press apparatus arises because the lubricant is present in a thin layer. The thin layer of lubricant does not have an adequate friction-reducing effect, and therefore, the force required in order for the press roll to drive the belt is large. Friction in the operation of the belt under these conditions also produces heat, which causes the temperature of the lubricant to rise, and its coefficient of viscosity to decrease. The decrease in viscosity of the lubricant results in a still further increase in friction.
For the above reason, various kinds of machine structures have been devised to supply more lubricant between the shoe and the belt in a shoe press apparatus. In addition, various kinds of belt structures have been invented in order to supply more lubricant to the press part.
FIGS. 3(a), 3(b) and 3(c) show a system, disclosed in U.S. Pat. No. 4,482,430, for supplying lubricant to the press part of a shoe press apparatus. FIG. 3(a) shows a lubricant feeder L on the upstream side in the machine direction relative to a shoe S, for supplying a lubricant L1 between a shoe S and a shoe-contacting surface B12 of a belt B1. A plurality of recesses B13 are provided in the shoe-contacting surface B12 of the belt B1 for holding lubricant. As the belt B1 advances under nip pressure while lubricant is held in the recesses B13, lubricant is supplied between the shoe S and the belt B1.
Various examples of recesses are disclosed in U.S. Pat. No. 4,482,430. For example, cup-shaped recesses B13 are shown in FIG. 3(b), and a groove-shaped recesses B13′ are shown in FIG. 3(c).
FIG. 4 shows a belt disclosed in unexamined Japanese Patent Publication 81291/1994. In this technology, a belt B2 comprises a base body formed by overlaying machine direction yarns B24 and cross machine direction yarns B25, and a high molecular weight elastic section provided on the base body. The belt B2 has a wet paper web-contacting surface B21 and a shoe-contacting surface B22.
Convex parts B23 are formed on the shoe-contacting surface B22 of the belt B2. These convex parts, in turn, provide recesses in the shoe contacting surface B22. This enables lubricant to be held in the shoe-contacting surface B22 of the belt B2, and supplied between the shoe and the belt B2.
Although the belts of FIGS. 3 and 4 have recesses in their shoe-contacting surfaces so that lubricant may be held therein, the supply of lubricant may still be insufficient since no lubricant is held on the surfaces of the convex parts which are in contact with the shoe. The object of the invention is to provide a belt for a papermaking machine which ensures a sufficient supply of lubricant between the belt and the shoe.